Solid state drives (SSDs) are storage media in which flash memories are utilized. Because flash memories suffer from the characteristic of degradation of elements caused by write-erase operations, SSDs configured from flash memories have an upper limit of the amount of write data. One indicator of the life time of an SSD defined by the upper limit of the amount of write data is a DWPD, for example. DWPD is an abbreviation for drive write per day, and is a value indicating how many times the entire capacity of an SSD can be overwritten every single day.
In order to extend the life time of an SSD having the upper limit of the amount of write data, a data compression technique to reduce the size of data written to the SSD or a data deduplication technique to reduce the data write cycle are used in storage apparatuses utilizing SSDs.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-108304
Patent Document 2: Japanese Laid-open Patent Publication No. 2015-201050
Patent Document 3: Japanese Laid-open Patent Publication No. 2015-204118
It is expected that technology that can enable higher capacities and less expensive costs, such as TLCs, will become mainstream technology for flash memories, rather than high-reliability and longer-lifetime SLCs and MLCs, although the upper limits of the amount of write data decrease. This is because a higher capacity of a drive can increase the writable capacity per day, even though the DWPD value, an indicator of the life time of the entire drive, is smaller. SLC is an abbreviation for single level cell, MLC is an abbreviation for multiple level cell, and TLC is an abbreviation for triple level cell.
The amounts of data accesses and read/write ratios of storage apparatuses, however, vary depending on workloads of tasks. Accordingly, in order to use SSDs in a reliable manner in a mixed workload environment in which a storage apparatus is used for multiple tasks, a technique to extend the life times of SSDs remains important even when high-capacity SSDs are now available.
Therefore, as a method to extend the life times of SSDs having small DWPDs, a development of a new access method that can reduce the amount of write data and allows once-written-data to be used fundamentally as read intensive is expected. Merely carrying out a compression process and a deduplication process of data, however, cannot ensure the data to become read intensive. For that reason, storing write intensive data to an SSD having a smaller DWPD, i.e., an SSD having a shorter life time, may shorten the life time of the SSD.